1. Field of the Invention
The present invention relates to a friction clutch, particularly for a motor vehicle, comprising differentiated friction means.
2. Description of the Related Art
In a motor vehicle, a friction clutch has the task of transmitting a torque between a rotating input element and a rotating output element, by virtue of the friction between a pressure plate and a flywheel.
In general, the input element is formed by a friction disc and the output element is formed by a hub which is designed in particular to be coupled to a gearbox input shaft.
The friction clutch also has the task of ensuring the continuity of the torque transmitted and of filtering out the vibrations coming from the engine, in particular by virtue of damping means which comprise elastic elements and friction means.
When the flywheel is coupled to the gearbox input shaft via the friction clutch, the gearbox input shaft is generally subjected to a torque in a first direction, referred to as the counter-clockwise direction. In this counter-clockwise direction, the torque acts in the same direction as the direction of rotation of the engine.
If necessary, the driver may bring about engine braking by releasing the accelerator of the vehicle. In this case, the gearbox input shaft is subjected to a torque in a second direction, referred to as the clockwise direction. In this clockwise direction, the torque acts in the opposite direction to the direction of rotation of the engine.
The torque in the clockwise direction is generally not as high as the torque in the counter-clockwise direction. This is why, in order to prevent locking of the damping means of the clutch in the case of clockwise torque, these damping means usually comprise friction means which, depending on the counter-clockwise or clockwise direction of the torque, ensure different levels of friction (high or low respectively).
There has therefore been proposed, in the prior art, a friction clutch, particularly for a motor vehicle, of the type comprising at least one damper having:                rotating input and output elements,        elastic elements with a circumferential effect carried by first and second guide washers which are connected in a rotationally fixed manner to a first of the input and output elements and by a web connected in a rotationally fixed manner to a second of the input and output elements,        friction means, which are activated by relative angular displacement of the guide washers and of the web and which comprise a friction element capable of being coupled to the web via complementary circumferential stops, referred to as coupling stops, which interact only when this web is displaced angularly in a predetermined direction with respect to the guide washers.        
Usually, the elastic elements with a circumferential effect are housed in openings provided in the guide washers and the web. In a clutch which is designed in particular to equip a heavy weight, three openings may be provided in the web, said openings being spaced apart in the circumferential direction.
The friction element is coupled to the web in the case of counter-clockwise torque. Thus, different levels of friction are obtained as a function of the counter-clockwise or clockwise direction of the torque to which the gearbox input shaft is subjected, that is to say a friction designed to bring about a first hysteresis in the counter-clockwise direction and a friction designed to bring about a second hysteresis in the clockwise direction.
In general, the friction element comprises a friction washer, the coupling stops being formed by coupling tabs which extend the outer contour of this friction washer. In this case, the coupling tabs are designed to cooperate with complementary coupling stops delimited by notches provided in the contours of the openings of the web. These tabs, which are housed between the notches and the elastic elements, are also driven by these elastic elements.
These notches make it possible to house the free ends of the coupling tabs between the elastic elements and the zones of the contours of the openings which form bearing points for the elastic elements.
However, the coupling notches constitute deficits of material which make the web more fragile.